Use rake to start the test suite in Jenkins

Originally created by @bertagaz on #12503 (Redmine)

While we worked on the vagrant build switch part of tails/tails#5630 (tails/sysadmin#11972 and friends), the Rakefile got improved with some code that can be used to start the test suite through rake test.

There’s been a pull request to use switch to this in Jenkins.

Related issues

• Related to tails/sysadmin#11972